(36c) Universal Trends in Catalytic Methane Activation in Presence of an Oxidizing Agent

Methane conversion technologies are an important aspect of both elimination of greenhouse gas emissions and utilization of natural gas and biomass derivatives. However, because of its strong C-H bonds, methane is hard to break down and convert to other higher valued molecules. A series of oxidizing agents have been used to activate methane, and despite methane activation being the common slow step, the reported optimal catalyst and reaction mechanism differ for each oxidizing agent.

Notwithstanding the stark differences in catalytic behavior, we have identified universal trends in catalytic methane activation in the presence of the different oxidizing agents (O₂, H₂O, CO₂, H₂O₂). Our study using density functional theory (DFT) and microkinetic modeling shows the importance of the harmony between oxidizing agent and catalytic metal, and highlights the required balance between the binding strength and surface coverage to achieve the maximum methane activation rate. Our work also successfully predicts the abnormal size dependence of Pd for methane partial oxidation in a study combining DFT calculations with Wulff construction and kinetic Monte Carlo (kMC) simulations. The interplay between different surface facets of a Pd particle has a pronounced effect on the dynamic formation of surface oxygen species, and in turn, methane activation. The diffusion of oxygen atoms from Pd(100) to Pd(111) is crucial for surface oxygen formation on the (111) facet, although the binding energies on these two facets from DFT calculations are nearly identical. Lowering the rate of oxygen diffusion by increasing the width of the (111) and (100) terraces in our model, correctly reproduces the experimentally observed trend of higher methane conversion rates with increasing nanoparticle size. Overall, our results provide fundamental insights into catalytic methane conversion, which can be leveraged for the design of improved catalysts.